1. Field of Invention
The present invention relates to a process for preparing a metallic porous body for use in electrodes for batteries, various filters, catalyst carriers, etc., particularly a process for preparing a metallic porous body suitable for use as an electrode substrate for an alkaline secondary battery, such as a nickel-cadmium battery, a nickel-zinc battery, and a nickel-hydrogen battery; an electrode substrate for a battery prepared using the metallic porous bodyland its production process.
2. Description of Prior Art
Storage batteries which have been used in various power sources are a lead storage battery and an alkaline storage battery. Of these batteries, the alkaline storage battery has been used widely in various types of portable equipment in the case of a small battery and in industries in the case of a large battery for reasons including that high reliability could be expected and a reduction in size and weight is also possible. In the alkaline storage batteries, zinc, iron, hydrogen, etc. besides cadmium are used as a negative electrode. On the other hand, the positive electrode is, in most cases, a nickel electrode although an air electrode or a silver oxide electrode has been partly accepted. Conversion from a pocket type to a sintering type resulted in improved properties of the alkaline storage battery, and that hermetic sealing has become possible and expanded the applications of the alkaline storage batteries.
In the conventional powder sintering system, when the porosity of the substrate is brought to not less than 85%, the strength is remarkably lowered, limiting the filling of an active material. This in turn limits an increase in the capacity of the battery. For this reason, in the case of a substrate having a higher porosity, i.e., a porosity of not less than 90%, a metallic porous substrate having a three-dimensional network structure, wherein pores are interconnected with one another, or a foam substrate or a fibrous substrate has been proposed instead of the sintered substrate and put to practical use. Such a metallic porous substrate having a high porosity has been prepared by a plating method disclosed in Japanese Patent Laid-Open No. 174484/1982 and a sintering method disclosed in Japanese Patent Publication No. 17554/1963. In the plating method, the surface of a skeleton of a foamed resin, such as a urethane foam, is coated with a carbon powder or the like to render the resin conductive, Ni is electro-deposited on the conductive surface of the resin by electroplating, and the foamed resin and carbon are then removed, thereby preparing a metallic porous body. According to this method, steps for the preparation of the metallic porous body are complicated although the resultant metallic porous body can satisfy strength and other requirements. On the other hand, in the sintering method, a slurried metal powder is impregnated into the surface of the skeleton in a foamed resin, such as a urethane foam, and the impregnated foamed resin is then heated to sinter the metal powder. Celmet comprising Ni metal (a product of Sumitomo Electric Industries, Ltd.) is already commercially available as the metallic porous body prepared by these methods and has been used as an electrode substrate for an alkaline secondary battery.
The application of the above conventional metallic porous body to an electrode substrate for a battery has contributed greatly to an increase in the capacity of the battery. In the case of an large alkaline secondary battery contemplated for use in electric cars or the like, however, the electrode substrate, for a battery, using the conventional Ni porous body, because of large electrode area, has high electrical resistance and, hence, causes a large voltage drop at high discharge rates, limiting the output from the battery. Further, potential distribution occurs within the plate face, resulting in lowered charging efficiency.